There are many situations in which it is necessary to measure the one-way delay incurred by messages that are transported between two sites coupled by a channel.
In the past, typically the measurement of a one-way delay through a channel has used a "round-trip" calculation approach. With such a method, typically a central site, A, would first record the initial time, and then send a predetermined signal through the channel that couples the central site to the remote site, B. At the B site, the channel would loop-back on itself, thus coupling the receive path to the transmit path. As a result, the signal would ultimately return to the A site. Upon receipt of the signal, the A site would note the elapsed time. The one-way channel delay would then be calculated by dividing the elapsed time by the value 2.
The problem with such an approach is that it does not take into account the asymmetries between the transmit and receive paths. Thus, it may be that the time needed for the signal to travel from A to B was greater than (or less than) the time for the signal to travel from B to A.
In most real-life applications, such asymmetries can be substantial. As a result, if the application depends on precisely measuring the one-way delay, the above round-trip approach can result in substantial errors.
One application that requires precise measurements of one-way channel delays is simulcast systems. The use of simulcast transmission to increase the effective coverage area of land-mobile radio systems is well known. In simulcast transmission, two or more transmitters, broadcasting the identical message simultaneously on the same frequency, are located so that coverage is available over a larger area than can be covered by the individual transmitters acting alone.
As is known, such simulcast transmission systems require that the message be transmitted from all transmitter sites at exactly the same time. If the message is transmitted by various transmitters at different times, this causes substantial distortion for subscriber units located in an over-lap area where the message is received from multiple transmitters. This distortion occurs when the various transmissions of the message arrive at a receiving subscriber unit with even slight phase or timing differences with respect to each other.
The fundamental problem with achieving proper simulcast operation is how to synchronize message transmissions at all transmitter sites. Typically the messages are formed at a central, or "prime" site, and then delivered by distribution channels to the remote sites for broadcast by the transmitters located there. To guarantee a time-precise arrival of messages at the remote sites, therefore, the delay introduced by the channels connecting the prime to the remote sites must be determined with precision.
In general, each channel will have a different delay. Once these individual delays are exactly known, then simultaneous transmissions of messages from all transmitters may be achieved by compensating for the variations in channel delays. Thus system-wide synchronization of all transmitters may be achieved only if the one-way delays for all channels can be precisely measured.
Accordingly, there is a need for an improved method for measuring channel delay.